Born in Lugdunum in the province of Gaul in 186, he was the son of the future emperor Septimius Severus and Julia Domna. His given name was Marcus Aurelius Antoninus but he was later given the nickname Caracalla, which referred to the Gallic hooded tunic he habitually wore and which he made fashionable.

Severus, who had taken the imperial throne in 193, died in 211 while visiting Eboracum (York), and Caracalla was proclaimed co-emperor with his brother Publius Septimius Antoninius Geta. Caracalla had Geta assassinated and carried out a vendetta against Geta's supporters, in order to strengthen his own hold on power. When the inhabitants of Alexandria heard Caracalla's claims that he had killed Geta in self-defense, they produced a satire mocking this claim, as well as Caracalla's other pretensions. Caracalla responded to this insult savagely in 215 by slaughtering the deputation of leading citizens who had unsuspectingly assembled before the city to greet his arrival, then unleashed his troops for several days of looting and plunder of Alexandria. According to historian Cassius Dio, over 20,000 people were killed.

During his reign as emperor, Caracalla raised the pay of an average legionaire to 675 denarii and lavished many benefits on the army, as instructed by his father Septimius Severus who had told him to always mind the soldiers and ignore everyone else.

Three things stand out from his reign: the edict of 212 (Constitutio Antoniniana) granting Roman citizenship to freemen throughout the Roman Empire in order to increase taxation; debasing the silver content in Roman coinage by 25% in order to pay the legions their bounties; and the construction of a large thermae outside Rome, the remains of which, known as the Baths of Caracalla, can still be seen.

Caracalla had effectively become a military dictator, and was consequently very unpopular except with the soldiers. While travelling from Edessa to begin a war with Parthia, he was assassinated while urinating at a roadside near Harran on April 8, 217 by Martialis, one of his attendants, who was immediately killed by an archer. He was succeeded by the Praetorian Prefect of the Guard, Macrinus.

Mythical king of Britain

Geoffrey of Monmouth lists Caracalla, named Bassianus in the Historia Regum Britanniae, as one of the kings of Britain following the death of Geta. This is partially true as Geta was well liked in the west when he was killed, and Caracalla probably exerted his power over the Britons harshly. In this account, Caracalla is listed as a half-brother of Geta through a Briton mother. This claim is highly criticized by historians. The text goes on to say that a general named Carausius was given ships to defend the British coastline and instead he rose up and defeated Caracalla; although it never states that Caracalla was killed in this battle, it does say that Caracalla fled from it. After this, Roman rule weakened considerably in Britain until it was fully restored by the Caesar Constantius Chlorus.